Protective circuit



Nov. 15, 1938. o. P. M CARTY 2,137,216

PROTECTIVE CIRCUIT Original Filed Jan. 6, 1937 4 T0 VOLTAGE SENSITIVECON7'/?0L FOR g Inventor: Orin P McCartg,

is Attorhegf Patented Nov. 15, 1938 PATENT ()FFICE.

PROTECTIVE CIRCUIT Orin P. McCarty, Pittsfield, Mass, assignor toGeneral Electric Company, a corporation of New York Application January6, 1937, Serial No. 119,234 Renewed March 17, 1938 15 Claims.

My invention relates to protective circuits and more particularly tomeans for protecting an impedance'which carries the main load current ofa transformer during operation under load of a tap changer on thetransformer.

Protective means of this type is disclosed and broadly claimed in myReissue Patent No. 20,147, dated October 27, 1936. The present inventionis particularly adapted for use withautomatic tap changing voltageregulators and boosters of the type disclosed and claimed in myapplication Serial No. 69,789, filed March 20, 1936 as acontinuation-in-part of an application filed April 16,

i 1935. This is a continuation-in-part of my application Serial No.47,425, filed October 30, 1935. The above referred to reissue patent andpatent applications are all assigned to the assignee of thisapplication.

When transformer taps are changed under load,

means are often provided for snapping the movable contact or contactsrapidly from one fixed contact or transformer tap to the next fixedcontact or transformer tap. In certain arrangements of this type animpedance is provided for carrying the current during the switchingoperation. Between switching operations the impedance is substantiallyshort-circuited by the closed contacts of the tap changing switch. Inthis way there is always a closed conducting path for the current and anobjectionable momentary break in the. circuit during the switchingoperation is eliminated.

As a normal tap changing operation of the above type takes placerelatively rapidly,'I have found that it is not necessary to provide animpedance which can carry the current for long periods or indefinitelyand it is only necessary to provide an impedance which can carry thecurrent safely and without overheating during the 40 relatively shortinterval of time during which the tap changing switch is going from onecontactmaking position to the next. This permits a substantial saving incost as an impedance which can carry the line current continuously ismuch more expensive and larger than an impedance which need only carrythe line current momentarily. It sometimes happens, however, that themechanism for driving the tap changing switch gets out of order so thatfor one reason or another the switch will stop in an intermediateposition between contact making positions. When this hap pens the loadcurrent flowing continuously through the impedance causes it to overheatto such an extent as to cause failure of the device and sometimes evenfire.

In accordance with my invention I provide improved means for protectingthe impedance from the' effects of a. stopping of the tap changingswitch in an off position. One feature of this means comprisesrelatively quick acting means for disconnecting andeffectively-by-passing the impedance when an ordinary value of linecurrent flow passes through the impedance for substantially longer thana normal tap changing operation combined with relatively slow actingmeans for short-circuiting the impedance in case the current flow is solow as to fail to cause operation of the first means and yet is highenough to cause eventual overheating of the impedance. Another featureis the provision of means for protecting the transformer against theeffect of said bypassing or short-circuiting means when an interruptedtap change is eventually completed.

An object of my invention is to provide a new and improved protectivemeans for a switching impedance.

Another object of my invention is to provide improved means fordisconnecting and by-passing or short-circuiting an impedance which isadapted to carry the line current during a transformer tap changingoperation, in the event of stopping the tap changing means in an oposition.

A further object of my invention is'to provide a new and improvedprotective circuit and appae ratus for a transformer tap changing underload type of. regulator.

My invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims. 7

In the drawing, Fig. 1 is a diagrammatic showing of an embodiment of myinvention and FI Z is a similar showing of a modification thereof.

Referring now to Fig. 1 of the accompanyin drawing, l is a switchingimpedance which is connected around a transformer tap changing switch 2.Switch 2 is arranged to change taps on a transformer winding 3, which isshown, by way of example as the series winding of an autotransformerhaving an exciting windin 4 nected across an alternating current circuit5. The winding 3 is connected in circuit 5 by means of an adjustableconnector 6 for selective engagement with the taps on the winding 3. Thepurpose of this is to vary the range of voltage adjustment produced bythe tap changer 2. In the midposition of connector 6, as shown in thedrawin the tap changer can give equal amounts of voltage buck and boostin the circuit 5. If the connector 6 is connected to one end of thewinding 3 the tap changing means will cause the autotransformer toproduce varying amounts of voltage buck, while if the connector 6 isconnected to the other end of the winding 3 the tap changing means willcause the autotransformer to produce varying amounts of voltage boost.With the connector 6 connected to a tap intermediate the midtap and theend of the winding 3 the buck and boost ratio will be 2 and 7 /g, orvice versa. The arrangement can be operated with the power flow in thecircuit 5 from right to left, as viewed in the drawing, or from left toright.

The details of the mechanism for driving the tap changing switch 2 formno part of my present invention and the movable contact may be operatedby hand if desired. However, the present preferred form of automaticmeans for driving the movable contact of the tap changing switch 2 inresponse to the voltage of the circuit 5 is disclosed in my copendingapplication, Serial No. 69,789, filed March 20, 1936, and assigned tothe assignee of the present application. Such anarrangement produces anautomatic voltage regulating device which is well adapted forautomatically holding substantially constant voltage on branch feedercircuits.

It is immaterial to my invention, in its broader aspects, that theimpedance I be of any particular type and it may be an ordinary resistoror a self-saturating iron core reactor, or a well known non-linearferrc-resonant circuit or net work of any suitable type. However, Iprefer to use a resistor of ceramic resistance material known as"Thyrite, the composition of which is disclosed and claimed in PatentNo. 1,822,742, granted September 8, 1931, on an application of K. B.McEachron and assigned to the assignee of the present application.Thismaterial has the property of markedly reducing its electricalresistance substantially instantaneously with increases in voltageapplied thereto or increases in current flowing therethrough. With sucha resistor its electrical resistance is relatively very high when thecontacts of the tap changing switch 2 are closed, as the voltage acrossthe resistor is then low. Consequently, the current through the resistoris very low resulting in minimum heating andminimum losses in theresistor. When the contacts of the tap changing switch 2 separate, thevoltage across the resistor l builds up and the current therethroughincreases. This causes its resistance to decrease greatly at the sametime so that the voltage drop across it is not as great as it would beacross an ordinary linear resistance. Consequently the switchingvoltage, that is, the voltage across the contacts of the switch 2, whenthey perform a tap changing operation, is held to a reasonably lowvalue. The above characteristic of resistor I is sometimes referred toas a negative resistance current characteristic.

For protecting the resistance I against the injurious effect ofcontinuous approximately full load line current flow therethrough in theevent of accidental stopping of the switch 2 in an off or intermediateposition, as indicated by the dotted position of the movable contact, Iprovide a switch having a fixed contact 1 and a movable blade contact 8which is biased towards fixed contact! by means of any suitable device,such as a spring 9. The switch is held open by means ,of a tension fuseI I].- The connections are such that fuse I0 is in series with resistorI, and the switch, when it closes, will by-pass, bridge 01' effectivelyshort circuit the normally conducting paths through the resistor I,switch 2 and series winding 3.

The resistor I isv shown in the shape of a hollow cylindrical memberthrough the center II of which is passed a bolt I2 carrying end nutsthereon for clamping the resistor between end conducting plates I3mounted on a suitable insulating support I4. During a switchingoperation the current flows through a conductor I5 to one of the platesI3 thence through the resistance material I to the other plate I3, fromthere through a conductor I6, through the tension fuse I0 and the bladeswitch 8 in series, and then through a conductor IT to the other side ofthe tap changing switch 2.

In normal operation, the fuse II) can carry the line current during theperiod of time required for an ordinary tap changing operation. If,however, the tap changing switch stops in an off" position the currentflowing through the fuse I 0 melts it, thereby allowing the spring 9 tocause closure of the switch by causing engagement between the bladecontact 8 and the stationary contact I. By this action the resistor I isautomatically disconnected and the normal current path through it isshort-circuited. It is to be noted that when the fuse I0 melts, themotion of the blade 8 issuch as to increase the distance between thefused or broken ends of the fuse I0 thereby effectively preventing anycontinuance of an are at this point.

It is difficult to provide a fuse which will not blow or melt whenmomentarily carrying full load line current during an ordinary tapchanging operation and which at the same time will melt when relativelylow current is passed therethrough continuously. Such relatively lowcurrent may, however, cause injury and overheating of the resistor I ifcontinued long enough. Consequently, there is provided suitable means inthe form of a small fusible metallic cylinder or ring I8 surrounding thebolt I2. This fusible means I8 responds to the temperature of theresistor I so that if the current flowing through the fuse I0 is lowenough not to cause it to melt but the current is still high enough tocause overheating of the resistor I then the fusible means I! will meltand in so doing will spread out and interconnect the conducting membersI3, thereby short-circuiting the resistor I. The fusible means I8 isrelatively slow acting compared to the relatively quick acting meansoperated by the fuse I0.

Experience has shown that vibration or sudden J'arring will sometimescause the switch 2 to complete a switching operation after it hasstopped in an "off" position. If this happens after the protectiveapparatus has by-passed or short-circuited any part of the serieswinding 3, a heavy short-circuit current will usually be induced in thatpart of the series winding 3 by the common winding 0. The one exceptionis when the movable contact 2 makes connection to the same tap thatconnector 6 is connected to for then no part of the series winding 3 isshortcircuited. If this short-circuit current persists the transformerwill be injured and its series winding will probably be burned out.

In the modification shown in Fig. 2, I provide means for protecting thetransformer against the above-described short-circuit current condition. This means consists of an auxiliary circuit maker and breakerfor deenergizing the shunt winding 4 of the autotransformer at timeswhen the series winding 3 of the autotransformer is short-circuited bythe protective means. In this way the production of any induced voltagein series winding 3 by transformer action iseliminated. A simple way toconstruct this auxiliary circuit maker and breaker and to secure itsproper operation is to combine it with the tension fuse-operated switch.Thus, as shown in Fig. 2, the upper terminal of the shunt winding 4 ofthe autotransformer is connected to an auxiliary contact l8 which isnormally engaged by the movable contact arm 8. When the fuse Ill meltsas a result of the stopping of the tapchanging switch 2 in an "off"position, the sprin 9 will cause the arm 8 to separate from the contactl8 thereby breaking the circuit through the shunt winding 4.

Aside from the rearrangement of the tension fuse-operated switch and itsadditional contact I8, Fig. 2 differs from Fig. 1 in a number of otherparticulars. The first is that the negative resistance currentcharacteristic impedance, the details of which are shown in Fig. 1 butmerely for simplicitys sake are not shown in Fig. 2, has a saturablecore reactor 19 connected in parallel therewith. For the range ofcurrent handled by transformer regulators of this type and with thepermissible voltage drop allowed in the by-pass impedance, a reactor andnegative characteristic resistor combination is less expensive than anequivalent resistor alone. This is because for equal current ratings,the reactor is less expensive than the negative resistor and with theparallel combination the reactor carries most of the current and thenegative resistor merely limits the high voltage peaks across-thereactor during times when the flux in the reactor core is belowsaturation values.

Another difierence between Fig. 1 and Fig. 2 is that in Fig. 2, theby-pass circuit which contains the fuse l and resistor l is connectedbetween the load circuit side of the tap-changing switch 2 and theelectrical midpoint of the series winding 3, whereas in Fig. 1 theby-pass circuit is connected across both the tap-changing switch 2 s andthe winding 3. The result is that in Fig. 2, the maximum continuousnormal voltage which can be applied to the impedance I is one-half thevoltage of the series winding 3 whereas in Fig. 1 when therange-adjusting terminal 6 is connected to either of the outermost tapsof winding 3, the full voltage of the series winding 3 can be appliedcontinuously to the by-pass impedance I.

Still another difference between the figures is that in Fig. 2, alightning by-pass 20 is connected between the mid-point of winding 3 andthe supply side of the circuit. The by-pass 20 is preferably made of thelightning arrester material Thyrite previously referred to. With such aconnection a high voltage surge, such as would result from a lightningstroke on tthe circuit 5, can readily pass through the negativecharacteristic resistors or impedances 20 and I in series as thesedevices will lower their resistances very markedly with increase involtage applied there-- to. Consequently the transformer windings areprevented from injury due to over-voltage as a result of such surges.

The switching duty imposed on the tension fuse-operated switch in Fig. 2is relatively greater than that in Fig. 1, because this switch nowactually removes a certain number of kilovolt am peres from the circuit.This switching principle is also somewhat different and may be describedas follows:

If, with the parts in the positions shown in the drawing and with thecircuit energized, the switch 2 is stopped in an off position, asindicated by the dotted line, the load current will be by-passed throughthe switch arm 8, tension fuse H), negative resistor l and reactor is inparallel, and the upper half of the series winding 3. When the fuse -Illmelts, the spring 9 carries the switch blade 8 toward contact 1, atthesame time opening the common winding excitation contacts at [8. Arcs arethen drawn at the melting fuse, and at the excitation contacts. Shouldthe are at the excitation contacts tend to inter-' .not exceed a valuewhich can easily be interrupted under oil. The excitation are readily interrupts and completely removes the regulator from the line.

In Fig. 2, there is also shown a tertiary winding 2| on the transformerfor supplying current to any suitable voltage sensitive control meansfor driving the tap-changing switch 2.

It should be understood that in practice, the negative resistance I ofFig. 2 can be constructed as in Fig. 1 with the fusible element f8 ifdesired. While I have shown and described particular embodiments of myinvention, it will be obvious to those skilled in the art that changesand modifications may be made in my invention and I,

therefore, aim in the appended claims to cover all such changes andmodifications as fali within the true spirit and scope'of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The combination, with an impedance for carrying the current duringoperation of a transformer tap changer under load, of protective meansfor said impedance comprising anormally open switch for short-circuitingthe current path through said impedance, means for biasing said switchclosed, and current responsive means for holding said switch open unlessthe current through said impedance exceeds a given value for a giventime.

2. The combination, with a circuit including an impedance for carryingthe current during operation of a transformer tap changer under load, ofprotective means for said impedance comprising a normally open switchfor bridging a portion of said circuit containing said impedance whensaid switch is closed, means for biasing said switch closed, and afusible element normally in circuit with said impedance for holding saidswitch open, said switch and fusible element be ing so arranged that theclosing of said switch increases the distance between the terminals ofsaid fusible element.

4. The combination, with an impedance for carrying the current duringoperation of a transformer tap changer under load, of protective meansfor said impedance comprising a relatively quick acting element forshort-circuiting the current path through said impedance when a currentabove a predetermined value flows through said impedance for a giventime, and an additional relatively slow acting element forshort-circuiting said impedance if a current of less than saidpredetermined value flows through said impedance for longer than saidgiven time.

5. The combination, with an impedance for carrying the current duringoperation of a transformer tap changer under load, of protective meansfor said impedance comprising a relatively quick acting currentresponsive element for shortcircuiting the current path through saidimpedance when a relatively high current flows through said impedancefor substantially longer than the time required by normal tap changingoperation,

and relatively slow acting means responsive to the temperature of saidimpedance for short-circuiting said impedance in case a current which isof lower value than is required to operate said relatively quick actingelement but-which is sufficient to cause injurious heating of saidimpedance continues to flow in said impedance.

6. Protective means for a load current by-pass resistor in a transformertap changer comprising, in combination, a normally open blade switchhaving a fixed. contact, a spring for urging the blade of said switchinto engagement with said fixed contact, a tension fuse electrically andmechanically connected to the blade of said switch for holding saidswitch open, a conductor connecting one side of said resistor to saidfixed contact, and a conductor connecting the other side of saidresistor to said fuse whereby a current can flow through said resistor,fuse and blade in series and upon melting bf said fuse by a relativelylong continued relatively high current said resistor will bedisconnectedand said switch will close thereby short-circuiting thecurrent path through said resistor and fuse.

7. In combination, regulating means including a single movable contactswitch for changing transformer tapsunderload,anegativeresistancecurrent characteristic resistor connected in shunt with saidmeans, and separate means responsive respectively to the effects ofrelatively high and low currents flowing continuously through saidresistor for effectively short-circuiting it.

8. In combination, an alternating current circuit, an auto-transformerconnected therein, a switch for changing taps on the series winding ofsaid autotransformer, a resistor whose resistance decreasesinstantaneously with increases in current therethrough connected tocarry the circuit current during operation of said tap changing switch,a normally open protective switch for short-circuiting the current paththrough said resistor, a spring tending to close said protective switch,a fusible element for holding said protective switch open against saidspring, said fusible element being normally connected in circuit withsaid resistor.

9. In an electric circuit, a pair of spaced contacts, an impedance,switching means having two operating positions closing a circuit aroundsaid contacts excluding and including respectively said impedance, andmeans responsive to current through said impedance above a given valuefor'a given time for causing said switching means to exclude saidimpedance.

10. In an electric circuit, a pair of spaced-contacts, an impedance,switching means having two operating positions for closing circuitsaround said contacts excluding and including respectively saidimpedance, means responsive to current through said impedance above agiven value for a given time for causing said switching means to excludesaid impedance, and means for shortcircuiting said impedance when saidswitching means is in a position to include said impedance. 11. Incombination, an alternating current closed circuit, a transformerwinding connected therein, means for changing taps on said winding, acurrent conducting by-pass path for carrying the load current of saidcircuit during a tap change on said winding, said by-pass comprising acommon portion connected to a movable switch contact and two parallelbranch portions, one of pair of normally separated contacts so connectedthat when they engage they complete a by-pass connection around saidimpedance, means for urging the contacts of said switch together, and afuse in circuit with said impedance for holding said switch contactsapart. said fuse being so proportioned as to ,melt when the tap changingmechanism of the regulator stops in an off position while saidalternating current circuit is carrying a substantial load currentwhereby said impedance will be open-circuited by the melting of saidfuse and substantially simultaneously therewith said protective switchwill close and complete a bypass connection around said impedance forsaid circuit current.

13. In combination, an alternating current load circuit, a transformertap changing switch. a.

transformer having a tapped winding adapted to be connected in saidcircuit by means of said switch, an exciting winding on saidtransformer, an impedance for carrying the circuit load current during atap changing operation, means for protecting said impedance againstoverloading should said tap changing switch stop in an intermediateposition. said protective means being operative to complete a relativelylow impedance short circuit around at least a portion of saidchangingoperation, and protective means for said impedance operative as a resultof thestopping of said tap changing means in an "021, positionwhen saidcircuit is carrying a substantial load current for substantiallysimultaneously closing a relatively low impedance by-pass connectionaround the normal current path through said impedance and deenergizingthe exciting winding of said transformer.

15. In combination, an alternating current load circuit, a voltageregulator for said circuit comprising an autotransformer connected insaid circuit and provided with means for changing taps on the serieswindings thereof, an impedance for carrying the circuit load currentduring a tap changing operation, and protective means for said impedanceoperative as a result of the stopping of said tap changing means in anoff position when said circuit is carrying a substantial load currentfor substantially simultaneously opening the circuit through saidimpedance, closing a relatively low impedance by-pass connection aroundthe normal current path through said impedance and opening the circuitof the common winding of said transformer.

ORIN P. McCARTY.

